1. Technical Field
The present disclosure relates bone anchors and, more particularly, to bone screw assemblies for use with surgical rods of varying diameter.
2. Background of Related Art
The adult human spinal column has 24 vertebrae coupled to one another by a tri-joint complex consisting of an anterior disc and the two posterior facet joints. The bones of the spinal column are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine is the top of the spine and includes the first seven vertebrae beginning at the base of the skull. The next twelve bones are the thoracic vertebrae. Thereafter, the next five bones form the lumbar vertebrae. Connecting to the lumbar vertebrae is the sacral bones, including the coccyx.
The spine is a highly flexible structure, capable of a high degree of translation and rotation about all three axes. Genetic or developmental irregularities, trauma, tumors, and disease, however, can result in spinal pathologies that limit the range of motion of the spine and/or threaten the critical elements of the nervous system housed within the spinal column.
Spinal fixation apparatuses, such as bone screws and rods, are widely employed in surgical procedures for correcting spinal injuries and diseases. Depending on the pathology and treatment, a surgeon will select the appropriate spinal rod for the particular procedure, e.g., the surgeon will select a spinal rod made from a specific material and/or having a specific cross-sectional diameter. The most common rod materials include titanium (e.g., Ti-CP) and its alloys (e.g., Ti-6Al-4V), cobalt-chrome alloy (CoCr), and stainless steel (SS). Rod diameters generally range from about 3.5 mm to about 7.0 mm. Typically, the selected spinal rod is then matched to a specific bone screw (or bone screws) according to the specific configuration of the selected spinal rod, e.g., the materials and/or cross-sectional diameter of the rod.
During implantation and fixation, the bone screws are first implanted into the respective vertebrae. Thereafter, the spinal rod is secured within each of the bone screws. However, unforeseen conditions, complications, or other intra-operative events may dictate a change in the previously selected spinal rod, e.g., a different diametered rod may be desired. As can be appreciated, removing the previously implanted bone screws is undesirable. Further, it may be desirable, in some procedures, to use a spinal rod having a different diameter along its length, e.g., a tapered rod. Thus, a need exists for a bone screw assembly capable of receiving spinal rods of different diameters and/or made from different materials.